Characteristics of subcooled water boiling on structured surfaces

Characteristics of the process and heat transfer of subcooled water boiling on mesostructured surfaces obtained by microarc oxidation of titanium foil with formation of a $\rm TiO_2$ layer and deposition of $\rm Al_2O_3$ particles from boiling nanofluid have been experimentally investigated. The experiments have been carried out in the forced flow of deaerated water in a vertical rectangular channel, $21 \times 5$ mm in size. The ranges of regime parameters are as follows: water mass velocity is up to $650$ kg/(m$^2$ s), subcooling is $30$–$75^{\circ}$C, pressure is $\sim10^5$ Pa, and heat flux rate is $0.7$–$5.0$ MW/m$^2$. It is established that the number of active nucleation sites is $(70$–$80) \times 10^5\,1$/(m$^2$ s) at the heat flux of $1.5$–$2.0$ MW/m$^2$. Significant subcooling of the liquid and good wettability of the structured surface provide intense deactivation and lead to random spatial distribution of the nucleation sites. The characteristic size of vapor bubbles is about $200$–$250\,\mu$m and the bubble lifetime is $200$–$500\,\mu$s. Application of the coating prepared by microarc oxidation enhances heat transfer by $20$–$30\%$. At high subcoolings of liquid, the characteristics of boiling on smooth surfaces and surfaces with the coating were fairly close.